For example, carbon has six protons and is atomic number 6. Carbon occurs naturally in three isotopes: carbon 12, which has 6 neutrons plus 6 protons equals 12 , carbon 13, which has 7 neutrons, and carbon 14, which has 8 neutrons. Every element has its own number of isotopes. Carbon is stable, meaning it never undergoes radioactive decay. Carbon is unstable and undergoes radioactive decay with a half-life of about 5, years meaning that half of the material will be gone after 5, years.
Isotopes have unique properties, and these properties make them useful in diagnostics and treatment applications. They are important in nuclear medicine, oil and gas exploration, basic research, and national security.
Isotopes are needed for research, commerce, medical diagnostics and treatment, and national security. Although the radioactive decay of a nucleus is too small to see with the naked eye, we can indirectly view radioactive decay in an environment called a cloud chamber.
Click here to learn about cloud chambers and to view an interesting Cloud Chamber Demonstration from the Jefferson Lab. We classify different types of radioactive decay by the radiation produced. The beta particle electron emitted is from the atomic nucleus and is not one of the electrons surrounding the nucleus. Such nuclei lie above the band of stability. Emission of an electron does not change the mass number of the nuclide but does increase the number of its protons and decrease the number of its neutrons.
Consequently, the n:p ratio is decreased, and the daughter nuclide lies closer to the band of stability than did the parent nuclide. Oxygen is an example of a nuclide that undergoes positron emission:. Positron emission is observed for nuclides in which the n:p ratio is low. These nuclides lie below the band of stability. Positron decay is the conversion of a proton into a neutron with the emission of a positron.
The n:p ratio increases, and the daughter nuclide lies closer to the band of stability than did the parent nuclide. For example, potassium undergoes electron capture:. Electron capture occurs when an inner shell electron combines with a proton and is converted into a neutron.
The loss of an inner shell electron leaves a vacancy that will be filled by one of the outer electrons. As the outer electron drops into the vacancy, it will emit energy. In most cases, the energy emitted will be in the form of an X-ray.
Electron capture has the same effect on the nucleus as does positron emission: The atomic number is decreased by one and the mass number does not change. This increases the n:p ratio, and the daughter nuclide lies closer to the band of stability than did the parent nuclide. Whether electron capture or positron emission occurs is difficult to predict. The choice is primarily due to kinetic factors, with the one requiring the smaller activation energy being the one more likely to occur. Figure 3 summarizes these types of decay, along with their equations and changes in atomic and mass numbers.
To perform a PET scan, a positron-emitting radioisotope is produced in a cyclotron and then attached to a substance that is used by the part of the body being investigated. How FDG is used by the body provides critical diagnostic information; for example, since cancers use glucose differently than normal tissues, FDG can reveal cancers. The 18 F emits positrons that interact with nearby electrons, producing a burst of gamma radiation. Different levels of gamma radiation produce different amounts of brightness and colors in the image, which can then be interpreted by a radiologist to reveal what is going on.
Unlike magnetic resonance imaging and X-rays, which only show how something looks, the big advantage of PET scans is that they show how something functions. PET scans are now usually performed in conjunction with a computed tomography scan. The naturally occurring radioactive isotopes of the heaviest elements fall into chains of successive disintegrations, or decays, and all the species in one chain constitute a radioactive family, or radioactive decay series.
Three of these series include most of the naturally radioactive elements of the periodic table. They are the uranium series, the actinide series, and the thorium series. The neptunium series is a fourth series, which is no longer significant on the earth because of the short half-lives of the species involved. Each series is characterized by a parent first member that has a long half-life and a series of daughter nuclides that ultimately lead to a stable end-product—that is, a nuclide on the band of stability Figure 5.
In all three series, the end-product is a stable isotope of lead. The neptunium series, previously thought to terminate with bismuth, terminates with thallium Radioactive decay follows first-order kinetics. Since first-order reactions have already been covered in detail in the kinetics chapter, we will now apply those concepts to nuclear decay reactions.
For example, cobalt, an isotope that emits gamma rays used to treat cancer, has a half-life of 5. Note that for a given substance, the intensity of radiation that it produces is directly proportional to the rate of decay of the substance and the amount of the substance. This is as expected for a process following first-order kinetics. Thus, a cobalt source that is used for cancer treatment must be replaced regularly to continue to be effective.
Since nuclear decay follows first-order kinetics, we can adapt the mathematical relationships used for first-order chemical reactions. We generally substitute the number of nuclei, N , for the concentration. If the rate is stated in nuclear decays per second, we refer to it as the activity of the radioactive sample. The rate for radioactive decay is:.
Example 1 applies these calculations to find the rates of radioactive decay for specific nuclides. Solution a The value of the rate constant is given by:. Or put another way, Substituting this into the equation for time for first-order kinetics, we have:.
How long will it take a sample of radon with a mass of 0. Through the use of radiometric dating, scientists can study the age of fossils or other remains of extinct organisms.
Boundless vets and curates high-quality, openly licensed content from around the Internet. This particular resource used the following sources:. Skip to main content. Atoms, Molecules, and Ions. Search for:. Learning Objective Discuss the properties of isotopes and their use in radiometric dating.
Key Points Isotopes are atoms of the same element that contain an identical number of protons, but a different number of neutrons. Despite having different numbers of neutrons, isotopes of the same element have very similar physical properties.
Some isotopes are unstable and will undergo radioactive decay to become other elements. The predictable half-life of different decaying isotopes allows scientists to date material based on its isotopic composition, such as with Carbon dating. Show Sources Boundless vets and curates high-quality, openly licensed content from around the Internet.
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